1. Field of the Invention
The invention relates to a method of processing a substrate for semiconductor device fabrication. More particularly, the invention relates to a method for improving film properties of a metal nitride/metal stack.
2. Description of the Background Art
In the fabrication of very large scale integration (VLSI) and ultra large scale integration (ULSI) integrated circuits, increasingly stringent demands are placed on the process capability and reliability of multilevel metallization techniques. Tungsten (W) has emerged as an alternative to aluminum (Al) in metallization techniques at various levels, including contact and vias.
An integrated barrier/liner structure is typically used to provide good adhesion between the metal conducting layer (W or Al) and the underlying material layer, as well as to prevent undesirable metal diffusion into the underlying layer.
These barrier/liner structures typically comprise refractory metal nitride/refractory metal combinationsxe2x80x94e.g., titanium nitride (TiN)/titanium (Ti), among others.
Titanium (Ti), for example, has been used as a glue or adhesion layer between silicon (Si) or silicon dioxide (SiO2) and a metal layer comprising Al or W. A barrier layer comprising, for example, TiN, is deposited upon the Ti adhesion layer prior to metal deposition to avoid metal diffusion into the underlying substrate.
Titanium nitride can be deposited by physical vapor deposition (PVD) as well as chemical vapor deposition (CVD).
However, CVD TiN may have an amorphous structurexe2x80x94e.g., when deposited from a metallo-organic titanium precursor, as opposed to the more orderly PVD Ti or PVD TiN layers. This difference in microstructure results in an integrated CVD TiN/PVD Ti stack having a weaker interfacial link than a PVD TiN/PVD Ti stack. Aside from weaker layer adhesion, structural discontinuity between the TiN and Ti layers also results in high inter-layer stress and interfacial defects.
Such a barrier/liner structure is often vulnerable to chemical and/or mechanical attack in subsequent processing steps such as W deposition, chemical cleaning and chemical mechanical polishing (CMP).
Furthermore, subsequently deposited aluminum may also diffuse through defects in the lattice or microstructure of the TiN/Ti stack to react with the underlying materials.
Therefore, a need exists for a process that will provide for an improved interfacial structure between a metal layer and a metal nitride layer that would prevent inter-metal diffusion, improves inter-layer adhesion, and improves chemical resistance during multilevel metallization processes.
The present invention is a method of forming a nitride layer on a metal layer, followed by modifying or treating the nitride and at least a portion of the underlying metal layer by exposing the nitride layer to a nitrogen-containing environment.
Metal nitride/metal stacks formed according to the embodiments of the present invention have improved properties such as enhanced adhesion, reduced interfacial stress and decreased resistivity. Such a structure, for example, is well-suited for barrier/liner applications in different metallization techniques for sub-0.18 xcexcm applications.
The nitrogen-containing environment may comprise gases such as nitrogen (N2) or ammonia (NH3), among others. Alternatively, the nitrogen-containing environment may also comprise hydrogen. The modification of the metal nitride/metal layers can be performed using plasma or thermal annealing. In one embodiment of the invention, a nitrogen-containing plasma is generated from a gas comprising a mixture of N2 and hydrogen (H2), or NH3.
The metal layer may comprise a refractory metal such as titanium (Ti), tantalum (Ta), tungsten (W), or combinations thereof, and may be deposited by either physical vapor deposition (PVD) or chemical vapor deposition (CVD). The metal nitride layer preferably comprises the same metal as the underlying refractory metal.
In one embodiment of the invention, titanium nitride (TiN) is deposited from a metallo-organic precursor. After TiN deposition, both the TiN layer and the underlying Ti layer are modified by exposing the layer stack to a nitrogen-containing environment for a sufficiently long time to allow active species to penetrate the TiN layer and reach the underlying Ti layer. When the as-deposited TiN layer is treated in an environment comprising both nitrogen and hydrogen, the resulting TiN layer exhibits a reduced impurity content and lower sheet resistance. According to the present invention, a thin nitrated-Ti layer is also formed between the treated TiN and Ti layers. This nitrated-Ti layer provides better lattice matching between the untreated portion of the Ti layer and the treated TiN layer, and leads to an integrated TiN/Ti structure with improved barrier characteristics and reduced inter-layer stress.
In another embodiment, a composite metal nitride layer is formed upon a metal layer by repeatedly depositing and treating relatively thin metal nitride layers for additional cycles, until a desired nitride thickness is obtained. For each plasma treating step, the entire uppermost nitride layer and a top portion of the underlying material layer are modified, resulting in changes in chemical composition and/or lattice structure. As a result, better lattice matching is obtained across the layer interface, leading to enhanced adhesion and reduced interfacial stress.
Optionally, the embodiments of the present invention may further comprise the step of treating the as-deposited metal layer in a first nitrogen-containing environment, prior to the deposition of a metal nitride layer. Such a treatment, for example, leads to the formation of a thin nitrated-metal layer, which provides better lattice matching between the untreated portion of the metal layer and the subsequently deposited metal nitride layer. As such, the resulting metal nitride/metal stack has enhanced adhesion and reduced stress. The as-deposited metal nitride layer is subsequently treated in a second nitrogen-containing environment, which may optionally comprise hydrogen. The treatment, which is preferably performed in a plasma, modifies both the metal nitride layer and at least a portion of the underlying nitrated-metal layer, resulting in improved film characteristics and interfacial properties.